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<h1>Historical Access Overview</h1>
<p>Historical Access is based on the original OPC HDA specification and 
 adapted to leverage the full benefits of OPC UA and are defined in <a 
	 href="http://www.opcfoundation.org/ua/part11"><?rh-udv_start name="UaSpec11" ?>UA Specifications Part 11 - Historical Access (http://www.opcfoundation.org/ua/part11)<?rh-udv_end ?></a>.</p>
<p>Intended for processing and analyzing data collected from a PLC/controller 
 and/or OPC DA Server.</p>
<p>In addition to historical data, the UA version of this specification 
 introduces the ability to obtain and process historical events information 
 too.</p>
<p>This technology provides the following capabilities:</p>
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		<td style="vertical-align: top;"><p style="text-align: center;"><span 
		 style="font-weight: bold;">Capability</span></p></td>
		<td style="vertical-align: top;"><span style="font-weight: bold;">Description</span></td>
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		<td style="vertical-align: top;"><p style="text-align: center; 
											 font-weight: bold;">Read</p></td>
		<td style="vertical-align: top;"><p>Obtain the history of 1...n 
		 Nodes in a single call. </p>
		<p>The request can specify numerous parameters such as the start/end 
		 dates, along with any statistical functions to invoke such as 
		 the Average, Min, Max, and many, many other <span style="font-style: italic;">Aggregates</span><span>.</span></p></td>
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		<td style="vertical-align: top;"><p style="text-align: center; 
											 font-weight: bold;">Update</p></td>
		<td style="vertical-align: top;"><p>Update the information stored 
		 in a database for 1...n Nodes in a single call. </p>
		<p>The request can specify the action, such as delete, update, 
		 or create new historical records. </p></td>
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		<td style="vertical-align: top;"><p style="text-align: center; 
											 font-weight: bold;">Subscription</p></td>
		<td style="vertical-align: top;"><p>The ability to request a Server 
		 to monitor Nodes and to send a notification to the Client when 
		 the value changes. Additional filters can be imposed such that 
		 the server can use an aggregate such as <span style="font-style: italic;">Average</span><span> 
		 and notify the client when values change etc.</span></p></td>
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<p>There are many other features of <?rh-udv_start name="TechnologyName" ?>Unified Architecture<?rh-udv_end ?> 
 that are inherited by the Historical &#160;Access <span style="font-style: italic;">profile</span>.</p>
<p>There are 4 demonstration applications available:</p>
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	<li><p>Historical Data</p></li>
	<ul type="disc">
		<li><p><a href="DAServerOverview.htm">Historical Data Server - 
		 Overview</a></p></li>
		<li><p><a href="DAClientOverview.htm">Historical Data Client - 
		 Overview</a></p></li>
	</ul>
	<li><p>Historical Events</p></li>
	<ul type="disc">
		<li><p><a href="HAEventsServerOverview.htm">Historical Events Server 
		 - Overview</a></p></li>
		<li><p><a href="HAEventsClientOverview.htm">Historical Events Client 
		 - Overview</a></p></li>
	</ul>
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